The present invention relates to modified phenolic resins, and also to processes for producing these. The present invention further relates to phenolic-resin formulations which comprise the modified phenolic resins, and also to processes for producing these. Finally, the present invention relates to specific uses of appropriate phenolic resins and, respectively, phenolic-resin formulations, and to the use of block copolymers for producing phenolic resins.
Phenolic resins are condensates of phenols with aldehydes, in particular with formaldehyde. Phenolic resins belong to the family of thermoset materials. In the hardened state, these have no melting point, because of three-dimensional crosslinking. Above a specific temperature threshold they begin to decompose. Phenolic resins are used as binders or glues or raw materials for producing molding compositions, electrical and thermal insulation materials, varnishes, frictional coatings, abrasives, timber-based materials, and fiber-based moldings. Phenolic resins are also used as foundry binders and refractory binders.
Significant application sectors for products produced with phenolic resins encompass the electrical, construction, and timber industry, vehicle construction, and also aerospace, the reason for this being that the materials have high strength, stiffness, and surface hardness, and also resistance to temperature change. Because of the low coefficient of thermal expansion, they are dimensionally stable and can withstand heating. Other features of phenolic resins are excellent fire- and flame-resistance, and also high electrical and thermal insulation capability.
In the synthesis of phenolic resin, the phenolic component and the aldehyde component react in a catalytic condensation reaction with elimination of water to give low-molecular-weight polymers. Decisive factors in determining the nature of a phenolic resin are not only the ratio of phenolic component to aldehyde component but also the selection of the catalyst. Acids are used as catalysts in the synthesis of novolaks. Bases are used for the resol condensation reaction.
However, a disadvantageous feature of the conventional phenolic-resin systems is that the phenolic resins often have a certain degree of undesired brittleness, therefore having only restricted extensibility.
WO 97/17385 discloses an arrangement for reducing brittleness, by modifying phenolic resins through incorporation of segments based on liquid rubbers. To this end, the liquid rubbers are reacted with formaldehyde or with derivatives thereof, and phenol or derivatives thereof, during synthesis of the phenolic resin. The result of this is chemical incorporation of the liquid rubbers into the structure of the phenolic resin. The rubber is generally incorporated here by way of a single bond at each of the two ends of the rubber molecule, and the two ends of the rubber molecules therefore bear functional groups, such as carboxy groups or amino groups.
The resultant phenolic resins are generally less brittle than the corresponding unmodified phenolic resins, since the lower density of the network results in a certain degree of flexibilization of the structure of the phenolic resin.
Examples of known liquid rubbers that can be used are the following materials containing functional groups: polybutadiene rubbers, acrylic butadiene rubbers, or silicone rubbers, and also vinylpyridine polymers or acrylic acid-butadiene-styrene polymers. Examples of functional groups in said liquid rubbers are hydroxy, carboxy, amino, or epoxy groups.
However, a disadvantage of said procedure is that the modification process has an adverse effect on the chemical and thermal aging of the phenolic resins. The strength of the resin can also decrease. Overall processability is impaired. The resin also has a very short shelf life.
There is also a process disclosed in JP 5770119 for phenolic resin production in which an intermediate is first provided, derived from liquid rubber and from an epoxy component. This then reacts in a second step with a precondensed phenolic resin. The intermediate reacts here with the phenolic resin primarily by way of its high content of reactive epoxy groups, apparent in terms of a measurable epoxy equivalent weight of about 1000 or less. This leads inter alia to further crosslinking of the phenolic resin, and this crosslinking in turn leads to very high viscosity of the precondensed phenolic resin. The high viscosity makes further processing of the phenolic resin more difficult; in particular during impregnation of textiles or of paper. The phenolic resin disclosed in said Japanese publication moreover lacks adequate shelf life. By way of example, phase separation is apparent after only a few days.
GB 2 075 517 A discloses a phenolic resin for which a liquid rubber is first reacted with a phenolic component, so that reaction with an aldehyde component can then give the phenolic resin. Polybutadienes are used as liquid rubbers here, and the phenolic component undergoes an addition reaction to the double bond of these.
The liquid rubbers therefore comprise aromatics which have random distribution across the polymer, where the liquid-rubber-polymer chains provide numerous possibilities of crosslinking in the context of phenolic-resin production. Here again, one problem inter alia is the increased viscosity of the resultant phenolic resin.
It is therefore an object of the present invention to mitigate at least one disadvantage of the prior art, and in particular to provide phenolic resins with reduced brittleness and therefore with increased extensibility, but in essence without any change in the aging properties. The phenolic resin should also have maximum end-use processability.